A holographic reconstruction system in the sense of this invention displays preferably moving three-dimensional scenes in real-time using video means by way of holographic reconstruction. The system comprises continuously controllable spatial light modulator means, which are encoded by a hologram processor with sequences of video holograms in order to spatially modulate waves of light which is capable of generating interference with holographic information. Thanks to the effects of light diffraction, the modulated light waves reconstruct object light points in an external reconstruction space in front of the eyes of observers, by way of local interferences, where said object light points optically reconstruct the desired three-dimensional scene. Light waves which represent the entirety of all object light points propagate in a directed manner towards the eyes of observers, so that one or multiple observers can watch those object light points in the form of the scene. In contrast to a stereoscopic representation, a holographic representation realises a substitution of the object.
In order to achieve a satisfying quality of the reconstruction, the observers should also be able to watch a reconstruction in a sufficiently large range of vision. Consequently, the reconstruction space must be as large as possible, and the size of the reconstructed scene should be at least 50 cm in diagonal, similar to TV and video representations.
However, it is disadvantageous that holographic reconstructions using large-sized light modulator means require for large diffraction angles a much higher resolution of the light modulator means than would be necessary for two-dimensional image representations, as is described by the known sampling theorem. This makes extraordinarily great demands on the hardware and software resources of the holographic reconstruction system—both as concerns the components for real-time provision of holographic information for encoding, and those for optical reconstruction of the scene.
Another known problem when reconstructing is an undisturbed propagation of the required light waves prior to generating interference. In order to reconstruct the object light points at the original position in space, and with the correct light point values, at least a part of the interfering light waves must arrive simultaneously at all the positions at which object light points are to be reconstructed through interference. This means that spatial coherence is required among as many as possible of the interfering light waves at each desired object light point.
Moreover, the wavelengths of the light waves which contribute to an object light point must not exhibit any uncontrolled path length difference among one another as caused by optical means.
In the description below, the term ‘optical axis’ denotes a straight line which coincides with the axis of symmetry of a reflecting or refracting optical element. Spatial light modulator means, which have been encoded by a hologram processor with holographic information of a three-dimensional scene, represent a ‘video hologram’. The interaction of a video hologram which is illuminated with coherent light with imaging means causes ‘a modulated wave’ to be generated. The imaging means define a ‘direction of propagation’ of the modulated wave. This direction of propagation can be modified by ‘optical wave tracking means’. If optical elements are disposed on the way to or if their effective direction is towards the video hologram, they will be referred to as ‘hologram-side’, and if they are disposed on the way to or if their effective direction is towards an eye position of an observer eye, they will be referred to as ‘observer-side’. A ‘visibility region’ describes a space which is disposed on the observer side at an eye position, and which represents the exit pupil of the system, and in which at least one observer eye must be situated for observing a holographically reconstructed scene. If, as is the case in the present application, an optical wave tracking means tracks the modulated wave to the current eye positions, the ‘tracking range’ defines the space which embraces all eye positions for which tracking is possible. In the technical literature on the subject, such a projection system is also known as a projection system with eye tracking.
The applicant of the present invention has already published several solutions for reducing the required resolution of the spatial light modulator means, for example in the international publication no. WO 2004/044659, titled “Video hologram and device for reconstructing video holograms”.
Those solutions are substantially based on one general principle. A wave which is spatially modulated with holographic information reconstructs the three-dimensional scene in a reconstruction space outside the system, said reconstruction space being positioned in front of one or both eyes of one or multiple observers. The geometry of the reconstruction space is defined on the one hand by the exit surface area of a display screen, through which the modulated wave leaves the reconstruction system, and, on the other, by the image area of a light source image, which forms a visibility region, also referred to as observer window, for at least one eye of one observer. Both surface areas define the geometry of a conical reconstruction space, while video holograms can also be encoded such that object light points do not only appear in front of, but also on and behind the display screen.
While the exit surface area of the display screen shall be as large as possible in order to achieve a large range of vision, the area of the visibility region can be reduced to the size of an eye pupil in order to efficiently use the resolution of the modulator means. The latter helps to keep the resolution of the modulator means low and thus to reduce the amount of information to be provided for holographic encoding.
From the geometrical description it becomes apparent that the reconstruction space shall preferably have a conical shape with an apex angle which is as large as possible, in order to be able to show large objects of a three-dimensional scene in their entirety as the distance between the observer and the reconstruction increases. However, a small visibility region may lead to problems with the visibility of the three-dimensional reconstruction, if the observer eye is only partly situated inside the visibility region. Already a slight movement of the observer may cause effects such as disappearance of visibility, vignetting or distortion of the spatial frequency spectrum. Moreover, the borders of the reconstruction space are difficult to find for an observer whose eyes are situated outside the visibility region. This is why the position of the reconstruction space is preferably adapted together with the visibility region and the position of the reconstruction itself to the new eye position if an observer moves. According to the known solution, the adaptation of the holographic reconstruction system to the eye position is executed by dislocating the illumination means which illuminates the light modulator means.
Because in a small visibility region the observer can see the holographic reconstruction with one eye only, a second wave, which is directed at the other eye, must provide a second reconstruction which differs in parallax. Because both reconstruction spaces must have the same base on the display screen in order to ensure perception of the two reconstruction spaces free from aberrations, their respective waves are spatially or temporally interleaved with the help of known autostereoscopic means. Spatial frequency filters and focussing means prevent optical cross-talking between the modulated waves. Such solutions have already been disclosed by the applicant in the aforementioned international patent application and in the application no. WO 2006/027228, titled “Method and device for encoding and reconstructing computer-generated video holograms”. If the reconstruction system is additionally meant to allow multiple observers to watch different reconstructions simultaneously, additional modulated waves will be required, typically two for each observer. These additional waves can be generated either in a space- or in a time-multiplex mode. However, the provision of additional waves will not be dealt with in this application.
In order to maintain a certain clarity, the description below relates mainly to the alignment of a single wave of the holographic system. The reconstruction system can modulate and direct further waves in analogy to the first one, if required. It appears to those skilled in the art that the idea of this invention can be applied as often as necessary for this, depending on the actual number of waves. When doing so, functional elements of the invention can preferably be used commonly for multiple modulated waves.
Light modulators as used in conventional video and TV projectors, with screen diagonals of few centimeters and smaller, are particularly suited for high-resolution, fast light modulation. In combination with the aforementioned geometry of the reconstruction space and small visibility regions, their small size also reduces the number of holographic cells which must be provided, addressed and encoded for each video hologram. This considerably reduces the computational load for each individual hologram, so that conventional, less expensive computing equipment can be used. Moreover, the illumination of the light modulator means with light which is capable of generating interference can be realised much easier, if the light modulator means are of smaller dimensions. In order to realise the aforementioned geometry of the reconstruction spaces, the reconstruction system is preferably designed as a projection system which optically enlarges the modulated wave prior to the reconstruction.
The international publication no. WO2006/119760, titled “Projection device and method for holographic reconstruction of scenes”, discloses a holographic projection system. This system will now be described in detail with reference to FIG. 1.
A plane wave LW with light which is capable of generating interference illuminates the entire surface area of a spatial light modulator SLM, which has a diagonal of no more than few centimeters, for example. In this embodiment, the light waves pass through a transmissive light modulator SLM. If the optical arrangement is modified accordingly, a reflective light modulator can be used instead as well. In any case does the modulator comprise modulator cells which are dynamically encoded by a hologram processor HP with holographic information of a desired three-dimensional scene. The encoded modulator cells thus represent a dynamic video hologram.
An optical projection system L projects the video hologram into an image plane IL0 on a focussing display screen S in an enlarged manner. A spatial frequency spectrum of the video hologram is thereby formed in the image-side focal plane of the optical projection system L, which is also referred to as the Fourier plane FTL.
Because of their matrix arrangement, the modulator cells modulate the wave spatially and equidistantly. As a consequence, multiple diffraction orders are simultaneously created in a periodic sequence in the Fourier plane FTL, which lie at different positions in a periodicity interval. The focussing display screen S would image all periodic sequences into the observer plane OL, and an observer would see them with an eye which is situated outside the visibility region, which is known as optical cross-talking. In order to avoid this, a spatial frequency filter AP in the form of an aperture mask is disposed in the Fourier plane FTL. Said mask prevents cross-talking by selecting one diffraction order, and the focussing display screen S only images the spatial spectral range of the modulated wave which has passed the spatial frequency filter AP into an observer plane OL at an eye position PE0. A visibility region for watching the reconstructed three-dimensional scene 3DS is thereby created at the eye position PE0. The image of the spatial frequency filter AP defines the geometry of the visibility region.
The diameter of the holographically encoded modulator cells, which are imaged on the display screen S, defines the other end of the reconstruction space.
In the example shown in FIG. 1, the display screen S is a lens. However, as explained above, the diameter of the display screen S should be very large compared with the size of the optical projection system L, so that the display screen can preferably also be a concave mirror.
In contrast to other known systems, this holographic projection system requires a special encoding of the modulator cells with the holographic information. The modulator cells are encoded with a video hologram such that the reconstruction of the three-dimensional scene 3DS through interferences only appears in that part of the light wave path where the enlarged and focussed wave has already left the reconstruction system through the display screen S. This allows optical path differences which may occur later during the propagation of light waves, e.g. due to different path lengths, to be taken into account already when encoding the modulator cells.
The described projection system also reconstructs the three-dimensional scene 3DS in a fix reconstruction space, and the scene will only be visible if one eye of the observer is situated in the visibility region, which is not physically visible. Unlimited mobility in front of the reconstruction system without loss or restriction of visibility of the holographic reconstruction will again be impossible with this projection system alone.
If an observer moves, position control means must track the reconstruction space and the modulated wave to the eye position of the respective observer eye such that the visibility region at the end of the reconstruction space always begins behind the eye position and the reconstructed scene always remains visible without any restrictions. For this, the projection system shown in FIG. 1 comprises an eye finder, known as such, which detects the exact eye position and which controls with the help of the position controller the visibility region to the new eye position. Such solution is known from patent document no. EP 0 946 066.
For a realistic holographic reconstruction, when tracking the modulated wave, also the holographic code provided to the modulator means can be adapted to the current eye position, because also in reality the viewing angles towards the spatial arrangement of objects of the three-dimensional scene and their visibility change if the observer position changes. Depending on the eye position, individual object details of the scene which are situated at various depths may or may not be visible due to varying overlapping of details and/or observer distances.
However, in a simplified holographic representation, an adaptation of the visibility of object details to the current eye position may be omitted.
Tracking by way of dislocating the entire reconstruction system is hardly feasible because of the size and weight of the display screen. The inventors have therefore already suggested in the German patent application no. DE 10 2006 024 092.8 to direct the modulated wave at the position of the corresponding observer eye with the help of an electronically controllable deflection unit, which is disposed in the vicinity of the focussing display screen. However, this requires great efforts as regards material resources and costs, because the deflection unit must be about as large as the display screen because it is disposed in the vicinity of the latter. If in contrast the deflection unit DFU is disposed closer to the projection lens L, as shown in FIG. 2, its size will be about the same as that of the projection lens L, and the deflection unit DFU can be built much smaller and thus more inexpensively. However, this requires a larger display screen S, as indicated in FIG. 2, because due to the inclination of the modulated wave towards an eye position PE1, the enlarged wave always only exits the reconstruction system through a limited section of the display screen S. A large section AO of the display screen S then always remains unused, because the exit position of the modulated wave varies as the eye position changes.
However, if the system has a small visibility region, so that each eye requires a separate modulated wave, it will be difficult to ensure with this solution that the two reconstruction spaces have the same base on the display screen.
FIG. 2 also shows that the deflection unit DFU prevents the effect that the image of the video hologram is not created directly on the display screen S. Instead, it lies near the display screen S in an inclined image plane IL1.
A satisfactory function of the deflection unit thus usually makes great demands on the optical elements of the reconstruction system. In particular, optical elements are required to have very large diameters, which means that in addition to a noticeable consumption of material, aberrations will occur which are difficult to correct.
The international application no. WO 2005/062106, titled “Projection apparatus for display of images floating in space” discloses a projection device for the display of two-dimensional images which float in space. That projection device comprises an image display, a pivoted planar mirror and a fix concave mirror. The document teaches that floating images will be displayed at a larger distance from the projection device if the concave mirror has an elliptic shape. The planar mirror can be pivoted at a right angle to the projection axis, in order to vary the angle of the main optical axis when the images exit the system. The distance, size and viewing angle of the floating images depend on the size of the elliptic mirror, on the positions of its foci, and on the location of the image display which is created through the interplay of the reflecting surfaces. Because of the different layout of the optical path of the wave modulated with two-dimensional image information, the aforementioned requirements cannot be fulfilled in the context of a holographic reconstruction.
The US-American application no. US 2005/0234348, titled “Apparatus for displaying images by projection on retina of viewer with eliminated adverse effect of intervening optics”, discloses a “retinal scanning display”, where an optical scanning system with a two-dimensionally pivoted scanner mirror and an elliptic projection mirror serially project an intensity- and phase-modulated laser light beam of the primary colours RGB each on to a retina of an observer eye. The image is reconstructed on the retina by way of serial pixel synthesis. The optical scanning system is disposed directly in front of an observer eye, and the scanner mirror with its two pivoting axes is disposed in one focal point of the elliptic projection mirror and the retina of the observer eye lies in the other focal point of the elliptic projection mirror. Because the image is composed of a serial sequence of laser light beams, i.e. of pixel by pixel and line by line of a scanned video image, that prior art solution is not suitable for holographic reconstruction through interference, because multiple light waves which would interfere are not simultaneously available. That solution does not use a spatial light modulator in the sense of the present description.